Multimode fibers are successfully used in high-speed data networks together with high-speed sources that are typically using transversally multimode vertical cavity surface emitting lasers, more simply called VCSELs. Multimode fibers operating at 850 nm and 1300 nm are well known.
Multimode fibers are affected by intermodal dispersion, which results from the fact that, in a multimode fiber, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information, but travelling with different propagation velocities. Modal dispersion is expressed in terms of Differential Mode Delay (DMD), which is a measure of the difference in pulse delay (ps/m) between the fastest and slowest modes traversing the fiber.
Typically, an optical fiber should have the broadest bandwidth, for it to be used in high bandwidth applications. For a given wavelength, the bandwidth may be characterized in several different ways. Typically, a distinction is made between the so-called overfilled launch condition (OFL) bandwidth and the so-called effective modal bandwidth condition (EMB). The acquisition of the OFL bandwidth attunes the use of a light source exhibiting uniform excitation over the entire radial surface of the optical fiber (using a laser diode or a Light Emitting Diode (LED)). The calculated effective modal bandwidth (EMBc) derived from the DMD measurement has been developed to estimate the minimum Effective Modal Bandwidth of the 50 μm core diameter fiber under inhomogeneous excitation over its radial surface, as it is when using a Vertical Cavity Surface Emitting Laser (VCSEL) source operating at 850 nm.
Embodiments of the method measuring DMD and calculating the effective modal bandwidth can be found in the FOTP 220 standard, while bandwidth measured over overfilled launch condition is described in IEC 66793-1-41 (TIA-FOTP-204).
In order to minimize modal dispersion, the multimode optical fibers used in data communications generally comprise a core, generally doped with Germanium, and showing a refractive index that decreases progressively going from the center of the fiber to its junction with a cladding. In general, the index profile is given by a relationship known as the “a profile”, as follows:
            n      ⁡              (        r        )              =                            n          1                ⁢                              1            -                          2              ⁢                                                          ⁢                                                Δ                  ⁡                                      (                                          r                      a                                        )                                                  α                                                    ⁢                                  ⁢        for        ⁢                                  ⁢        r            ≤      a        ,where:    n1 is a refractive index on an optical axis of a fiber;    r is a distance from said optical axis;    a is a radius of the core of said fiber;    Δ is a non-dimensional parameter, indicative of an index difference between the core and a cladding of the fiber; and    α is a non-dimensional parameter, indicative of a shape of the index profile.
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is thus possible to theoretically obtain a group velocity, which is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength.
In practice, however, multimode fibers are manufactured with a graded index central core surrounded by an outer cladding of constant refractive index. Thus, the core of the multimode fiber never corresponds to a theoretically perfect alpha profile, because the interface of the core (having an alpha profile) with the outer cladding (having a constant refractive index) interrupts the alpha profile. The outer cladding accelerates the higher order modes compared to the lower order modes and some differences of time delay appear within the highest order mode groups. This phenomenon is known as the cladding effect. In DMD measurement the responses acquired for the highest radial positions (i.e. nearest the outer cladding) exhibit multiple pulses, which results in a temporal spreading of the response signal. Therefore bandwidth is diminished by this cladding effect.
Graded-index alpha-shape profile and core-cladding interface of the multimode fibers are optimized to operate with GaAs VCSELs that can be directly current-modulated to support 10 Gbps and 25 Gbps systems at 850 nm. Backwards compatibility for use at 1300 nm with LED sources is also guaranteed for most of the 50 μm and 62.5 μm multimode fibers currently in use. The performances of such laser-optimized, high bandwidth 50 μm multimode fibers, also called OM4 fibers, have been standardized by the International Standardization Organization in document ISO/IEC 11801, as well as in TIA/EIA 492AAAD standard.
However, the explosion in demand for bandwidth in enterprise networks is driving an urgent need for higher Ethernet network speeds. To further increase the data bit rate for next generation 400 GbE systems, the use of InGaAs VCSELs operating at 40-56 Gb/s between around 1060 nm appears as a promising solution, as it will allow achieving higher speed with higher reliability, lower operating temperature and lower cost of the VCSELs. Furthermore, at this wavelength, the fiber exhibits lower attenuation, lower chromatic dispersion and higher potential modal bandwidth because of fewer modal groups if the graded-index alpha-shape profile is optimized to operate at this specific wavelength.
While such VCSELs can be proposed now for high-speed applications, fibers optimized for these VCSELs operating at wavelength greater than 950 nm are missing.
Actually, the one skilled in the art knows well that the inter-modal dispersion can be reduced by adapting the alpha-shape profile and the core-cladding interface to the operating wavelength.
The alpha value can be easily assessed by testing different alpha-shape profiles, the optimum alpha varying monotonically with wavelength for a given composition, whatever the core radius and the core index. However, defining the optimum geometry for the core-cladding interface is more delicate since there is no simple relationship between wavelength and the geometry of core-cladding interface whatever the core radius and core index.
At wavelengths longer than 850 nm, because of fewer modal groups, the proportion of modal groups directly affected by the core-cladding geometry is larger. Thus, its optimization is more delicate and its impact on the total bandwidth is increased. In the same manner, with small core radius, because of fewer modal groups, impact of core-cladding geometry on the total bandwidth is increased too.
Many studies have been carried out so far, in order to design multimode fibers, which bandwidth would be sufficiently high over a relatively large wavelength range.
Document EP 1 503 230, in the name of DRAKA Comteq BV, discloses a multimode optical fiber having a refractive index profile, comprising a light-guiding core surrounded by one or more cladding layers. According to this document, the multimode optical fibers can be obtained by using two or more dopants for building up the gradient index core, notably by using co-doping with fluorine F and germanium GeO2. By varying the concentration of dopants over the core radius, the intermodal dispersion characteristics of the multimode optical fiber can be adapted in such a manner that the bandwidth is less wavelength-dependent.
A drawback of this technique is that the bandwidths achieved with such multimode fibers and reported in this prior art document are not large enough.
Document EP 2 482 106, also in the name of DRAKA Comteq BV, discloses a multimode optical fiber, which includes a central core having a graded-index profile with a delta value of about 1.9 percent or greater. The graded-index core profile has at least two different alpha parameter values along the core radius, namely a first value in an inner zone of the central core and a second value in an outer zone of the central core. The second alpha parameter value is typically less than the first alpha parameter value. The graded-index core profile and its first derivative are typically substantially continuous over the width of the graded-index core.
Fibers disclosed in this prior art document show high numerical aperture NA with bandwidths optimized for a single wavelength at 850 nm. Moreover, the high NA value requires a graded-index core with two or more a-values.
Document U.S. Pat. No. 7,315,677 discloses multimode optical fibers comprising Germania (GeO2) and Fluorine co-doped in the core of the fiber. The dopant concentration profiles are defined by a pair of alpha parameters, α1 and α2. The operating window, or bandwidth window, is enlarged and attenuation, or loss, is low. In some embodiments, two operating windows are available for transmission.
Document U.S. Pat. No. 7,315,677 hence teaches “double alpha profiles” based on co-doping, with fluorine F and germanium GeO2; each dopant profile exhibits its own alpha. Such profiles are difficult to produce from a process point of view. Actually, the concentration shape of Ge and F are difficult to control.
Document U.S. Pat. No. 7,903,918 discloses bend resistant optical fibers, which are multi-moded at 1300 nm and include a core, an inner cladding, a low index ring and an outer cladding. The core has a graded index of refraction with a core alpha profile where 1.9≦α≦2.1, a maximum relative refractive index percent Δ1Max%, and a numerical aperture NA greater than 0.23. The inner cladding surrounds the core and has a maximum relative refractive index percent Δ2Max%, a minimum relative refractive index percent Δ2min%, and a radial thickness microns, wherein Δ1Max%>Δ2Max%. The low index ring surrounds the inner cladding and has a relative refractive index percent Δ3%, a radial thickness of at least 0.5 microns, a profile volume with an absolute magnitude of greater than 50%-μm2, wherein Δ2Min%≧Δ3%. The outer cladding surrounds the low index ring and has a relative refractive index percent Δ4%, such that Δ1Max%>Δ4%≧Δ2Max%.
Fibers disclosed in this document show high NA values, deep trenches and high trench volumes. They are optimized to operate at 1300 nm.
Document US 2010/0303428 discloses bend resistant multimode optical fibers, which comprise a core region and a cladding region surrounding and directly adjacent to the core region, the cladding region comprising a depressed-index annular region; the inner boundary of the depressed index region is an extension of the graded index core, the depressed region having a moat volume greater than 105%-μm2. Such fibers hence show deep trenches and high trench volumes.
Document US2013/0077926 discloses several embodiments of multimode fibers, in which trenches are added for bend-loss improvement. According to some embodiments, a multimode optical fiber comprises a graded-index glass core with refractive index Δ1, a maximum refractive index delta Δ1MAX, and a core radius between 10 and 40 microns; and cladding region surrounding the core comprising refractive index Δ4, wherein the fiber exhibits an overfilled bandwidth at an operating wavelength in a 900 to 1250 nm wavelength range of greater than 2.5 GHz·km. According to some embodiments, the fiber exhibits an overfilled bandwidth at a wavelength between 950 and 1100 nm, which is greater than 4 GHz·km. According to some embodiments the fiber exhibits an overfilled bandwidth at a wavelength between 950 and 1100 nm, which is greater than 10 GHz·km. The volumes of the trenches disclosed in this document are very large. Moreover, the profiles disclosed are quite complex to design.
Document US2013/0039626 also discloses multimode fibers in which a trench is added in the cladding, in order to improve the bending performances of the multimode fiber. The disclosed multimode optical fiber includes a graded index glass core having a diameter in the range of 24 microns to 40 microns, a graded index having an alpha profile less than 2.12 and a maximum relative refractive index in the range between 0.6% and 1.9%. The fiber also includes a cladding surrounding and in contact with the core. The cladding includes a depressed-index annular portion. The fiber further has an overfilled bandwidth greater than 2.0 GHz-km at 1310 nm.
The volumes of the trenches disclosed in this document are very large. Trench is added to improve the bending performances of small core diameter MMFs.
In view of the foregoing, it would be desirable to design a multimode optical fibre adapted to high-speed applications (next generation 400 GbE systems with VCSEL transmitting at 25 Gb/s or higher) and showing improvements over the prior art.
More precisely, it would be desirable to design such a multimode optical fiber, showing an OFL-bandwidth above 10000 MHz·km at an operating wavelength between 950 nm and 1310 nm.
It would also be desirable to design multimode optical fibers with optimized core-cladding geometry according to the number of mode groups supported by the MMF at the operating wavelength.